Method and evaporation chamber for generating a continuous vapor stream containing a compound having monovalent gallium therein, and a vacuum coating apparatus

ABSTRACT

In a method and evaporation chamber for generating a continuous vapor stream containing a compound in which gallium is present in monovalent form of a vacuum coating method for vacuum coating a substrate, an evaporation substance containing gallium in bivalent or trivalent form, is arranged together with metallic gallium in the evaporation chamber, that is closed on all sides and has a vapor exit opening. The evaporation substance is evaporated, and the vapor is brought into contact with the metallic gallium, causing the bivalent or trivalent gallium to be reduced to monovalent gallium in a vapor stream which subsequently exits in the direction of the substrate via the vapor exit opening.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and evaporation chamberfor generating a continuous vapor stream containing a compound in whichgallium is present in monovalent form in a vacuum coating process forvacuum coating a substrate. The invention also relates to a vacuumcoating apparatus operating according to such a method and having suchan evacuation chamber.

[0003] 2. Description of the Prior Art

[0004] Monovalent gallium (Ga¹⁺) is frequently used as a dopingsubstance in X-ray absorber materials such as CsBr or RbBr for storagephosphors. Substrates coated therewith are needed as radiation detectorsfor radiographic applications, for example. The monovalent gallium isapplied as GaBr, but GaBr cannot be chemically isolated. To achieve adoping despite this fact, it is suggested in U.S. Pat. No. 5,736,069 touse GaCl as the starting material, which is mixed with metallic Ga andwith the X-ray absorber material CsBr. This mix is dried at 400° C.,after which a monocrystal of the phosphor is grown at high temperature(e.g. 925° C.). The powder-type phosphor is obtained by milling themonocrystal, and is dispersed in a solvent (e.g. ethyl acetate) in asolution of a binder (e.g. polyethyl acrylate), and is layered onto thecarrier film, for instance by pouring, rolling or sedimenting.High-quality layers cannot be produced in this way. Good X-ray absorberlayers are usually produced in the context of a vacuum evaporatingprocess; that is, the X-ray absorber material is first evaporated in anevaporating device and then settles on the carrier. The high vaporpressure of the required starting materials, namely the bivalent ortrivalent gallium-halogenide compound and the gallium metal, acts toinhibit deposition of the doping substance containing monovalentgallium, which can be GaBr or Gal, by means of a vacuum coating method,since these would evaporate abruptly in the vacuum at common evaporatingtemperatures in the range of a few hundred degrees upon the formation ofthe dopant, even before the reaction of the starting materials hasbegun. As a result, it has not been possible to perform doping withmonovalent gallium in the context of a vacuum coating, and particularlyto simultaneously generate the X-ray absorber layer and to introduce thedoping in the context of an evaporation process.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to provide a method thatenables the generation of a continuous vapor stream containing amonovalent gallium doping substance even in a vacuum, despite the abovedescribed problems, so that a vacuum doping is possible.

[0006] This object is achieved in accordance with the invention in amethod for generating a continuous vapor stream containing a compound inwhich gallium is present in monovalent form in the context of a vacuumcoating method for vacuum coating a substrate, wherein a multivalentgallium-containing evaporation substance (i.e., an evaporation substancecontaining gallium in bivalent or trivalent form) is provided togetherwith metallic gallium in an evaporating vessel or chamber that is closedon all sides and that has a vapor stream exit opening. The evaporationsubstance is evaporated in this evaporating chamber, and the vapor isbrought into contact with the metallic gallium, causing the bivalent ortrivalent gallium to be reduced to monovalent gallium, whichsubsequently exits in the direction of the substrate via the vaporstream exit opening.

[0007] In the inventive method, the reaction components—that is, theevaporation substance consisting of a gallium compound, particularly agallium halogenide in which the gallium is present in bivalent ortrivalent form, as well as the metallic gallium—are arranged in a closedevaporation chamber, where the evaporation of the evaporation substanceand the actual reaction occur. At the chamber, only a very small vaporstream exit opening is provided, via which the vapor stream containingthe doping substance (for instance the GaBr) can exit. Since thereaction substances are present in a closed chamber containing only asmall opening, an abrupt evaporation is prevented, despite thetemperature, since the pressure relation prevailing in the interiorprevents a complete evaporation. The reaction of the vaporousevaporation substance and the metallic gallium which takes place in theclosed reaction space can be sufficient to enable the generation of acontinuous vapor stream containing the doping substance, which exitsinto the vacuum chamber and in the direction of the substrate via theexit window. The inventive method thus makes it possible to continuouslygenerate the dopant vapor stream inside a vacuum coating apparatus inwhich there is a vacuum, in the context of a vacuum coating process.

[0008] The evaporation substance can be inventively arranged in theevaporation chamber on a first level, and the metallic gallium can bearranged on a second level situated over the first level, so that theevaporation substance is evaporated separately from the metallic galliumbut comes into contact with it and reacts with it inside the chamber.Alternatively, the evaporation substance can be inventively mixed withthe metallic gallium, so that the evaporation substance is evaporated inthe metallic gallium, and an immediate reaction occurs. The reactiontemperature for the reduction lies between 250° C. and 950° C.,particularly between 300° C. and 900° C. The higher the temperature, themore complete the reaction; the reaction temperature at which thevaporous evaporation substance and the metallic gallium react shouldpreferably be about 500° C. In the embodiment wherein the evaporation ofthe evaporation substance occurs separately from the metallic gallium,in order to prevent the substance from evaporating too rapidly, atemperature gradient from 300° C. to 500° C., particularly from 350° C.to 450° C., is inventively produced between the metallic gallium and theevaporation substance, for which purpose the evaporation substance canbe inventively cooled and/or the metallic gallium can be heated. In thisway it is also possible to cool the evaporation substance for loweringthe vapor pressure, while allowing adjustment of the temperature in therange in which the reaction takes place.

[0009] In a further embodiment of the invention, the evaporation chamberis cooled in the region of the vapor exit opening, which likewiseresults in a local lowering of the pressure at that location. As aresult, the expansion of particulate vapor which takes place as thevapor exits through the vapor exit opening is “weaker” in the transitioninto the vacuum; so that fewer particle collisions arise in thisexpansion; thereby allowing the vapor particles to travel more linearlyin the direction of the substrate. A drifting apart and “smearing” ofthe vapor stream thus can be counteracted and a relatively sharper (moreconfined) vapor stream can be generated in the direction of thesubstrate. To both enable both a build-up of the required pressurerelations and generation of a sufficiently sharp vapor stream jet, it isappropriate to use an evaporation chamber with a vapor stream exitopening having a diameter between 2 μm and 2 mm, particularly between 5μm and 1.5 mm, preferably between 10 μm and 2 mm. Several such smallvapor exit openings can be provided. The evaporation chamber shouldconsist of a material that does not react with the evaporation substanceand/or with the metallic gallium or which cannot be wetted by thesematerials, such as graphite, aluminum oxide or boric nitride. In afurther embodiment of the invention, the evaporation substance can beevaporated before, during or after the evaporation of a coating materialwith which the substrate is being coated, particularly an X-ray absorbermaterial. That is, the X-ray absorber material can be evaporated firstfollowed by evaporation of the doping substance in a common evaporationapparatus. A reverse sequence of evaporation is also possible, and asimultaneous evaporation along with a coating of the substrate with theevaporation compound containing gallium in monovalent form and with theX-ray absorber material is particularly expedient.

[0010] The invention is also directed to an evaporation chamber forevaporating a substance, particularly one containing gallium, in thecontext of a vacuum coating process, this chamber being suitable for usein the method described above. This evaporation chamber inventively hasat least one space for accepting at least one evaporation substance andat least one other substance that acts chemically on the evaporationsubstance, and it can be or is closed on all sides, and has a vaporstream exit opening with a diameter of less than 2 mm, particularly lessthan 1 mm, provided at least one side.

[0011] Two spaces can be provided in the inventive chamber which aresituated one above the other and which are separated by avapor-permeable dividing wall, the lower space being provided foraccepting the evaporation substance, and the upper space being providedfor accepting the additional substance. The dividing wall can beinventively formed by a wire grating or a grid of holes or the like, orby a lid-like cover having one or more openings. The dividing wallconsists of a material that does not react with the additional substanceand/or with the substrate vapor and/or cannot be wetted by these. If theupper space is suitable for accepting the metallic gallium, inparticular, then the dividing wall can inventively consist of graphiteor aluminum oxide.

[0012] At least one heater can be additionally provided in accordancewith the invention for heating a region of the chamber, preferablydisposed in the region of the base of the chamber, such as the base ofthe lower space, or in the region of the dividing wall. At least onecooling arrangement for cooling a region of the chamber can beadditionally provided, in the upper chamber region, particularly in theregion of the vapor exit opening. The upper space itself can be closablewith a lid-like cover containing the vapor exit opening. The chamberitself can be made of graphite, aluminum oxide, or boric nitride, as canthe upper lid-like cover, as warranted.

[0013] The invention also relates to a device for vacuum coating asubstrate with an evaporation material, having at least one evaporationchamber of the above described type.

DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic illustration of a device for vacuum coating,as known in the art.

[0015]FIG. 2 is a side sectional view of a first embodiment of anevaporation chamber in accordance with the invention.

[0016]FIG. 3 is a side sectional view of a second embodiment of anevaporation chamber in accordance with the invention.

[0017]FIG. 4 is a side sectional view of a third embodiment of anevaporation chamber in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]FIG. 1 depicts a known apparatus 1 for vacuum coating, which isindicated only in an exemplary schematic form. The apparatus includes avacuum housing 2, in which the substrate 3 that is to be coated can bearranged. A heater 4 is allocated to this substrate 3, with which thesubstrate 3 can be heated. A first evaporation chamber 5 with anallocated heater 6 is also provided, in which the X-ray absorbermaterial that is to be deposited on the substrate 3 is contained and canbe evaporated. An additional evaporation chamber 7 is also provided, towhich a heater 8 is likewise allocated. In this evaporation chamber 7,the reaction components for generating the dopant vapor stream, which issubsequently deposited on the substrate 3 as well, are located. Acontrol unit 9 and a pump 10 are also depicted, the control unit 9serving to control all the heaters 4,6,8 as well as to control the pump10 by means of which the vacuum is created in the vacuum housing 2. Tothe extent described below, components of this conventional arrangementcan be used with the inventive embodiments described below.

[0019]FIG. 2 depicts an inventive evaporation chamber 7′ in a firstembodiment. This evaporation chamber 7′ has a housing 11, which ispreferably made of boric nitride, aluminum oxide or graphite. In theexample, an evaporation substance 12, for instance GaBr₂ or GaBr₃, andan additional substance 13, for instance metallic gallium, are arrangedin the interior of the evaporation chamber 7′. In the example, a heater14 is provided in the region of the base, for heating the materials 12,13. This leads to the evaporation of the evaporation substance 12 andthe vaporized substance comes into direct contact with the additionalsubstance 13 and reacts therewith for generating a dopant compound.

[0020] For example, if GaBr₂ or GaBr₃ is used as the evaporationsubstance, and metallic gallium is used as the additional substance,then the vaporized evaporation substance is reduced from the metallicgallium, as follows:

2 GaBr₂ (gaseous)+2 Ga (liquid)→4 GaBr (gaseous)

2 GaBr₃ (gaseous)+4 Ga (liquid)→6 GaBr (gaseous)

[0021] The GaBr contained in the dopant vapor which is generated in thisway exits into the vacuum region of the device housing 2 in thedirection of the substrate 3 via an evaporation exit opening 15, whichis formed at a lid-like cover 16 that can be placed onto the housing 11.The diameter of the evaporation exit opening preferably is in the rangebetween 10 μm and 1 mm. The lid-like cover 16 is also made of the abovedescribed material. It has two other functions besides closing thechamber housing 11. First, by means of the vapor exit opening istherein, the vapor current is channeled and its volume is limited, sincethe amount exiting from the interior of the evaporation chamber 7′depends on the diameter of the opening 15. Second, the cover 16 alsoserves to protect of the substrate against splashing of metallic galliumin the release of the GaBr gas.

[0022]FIG. 3 depicts another embodiment of an evaporation chamber 7″.This has a horizontal dividing wall 17, which divides the interior ofthe chamber 7′ into two spaces 18, 19. The evaporation substance, i.e.the GaBr₂ or GaBr₃, is located in the lower space 18, and the additionalsubstance, i.e. the metallic gallium, is located in the upper region.The dividing wall is permeable to the substrate vapor of the evaporationsubstance, so that it can react with the upper substance. To this end,the dividing wall 17 is constructed as a wire grating or as a perforatedshelf, and it has a series of openings, the diameter of which preferablyis between 0.5 and 5 mm. If metallic gallium is placed in the upperspace 19, then its high surface tension prevents dripping through theholes of the dividing wall 17 into the lower region 18. The dividingwall 17 should of a not react with any of the substances located in theevaporation chamber or be wetted by them. If the aforementioned galliumsubstances are used, then aluminum oxide or graphite are suitable forthis wall 17. Otherwise, the chamber construction corresponds to thataccording to FIG. 2.

[0023]FIG. 4 depicts a third embodiment of an inventive evaporationchamber 7″. This embodiment also has the multi-chamber constructionshown in FIG. 3. In this embodiment, a cooling arrangement 20 isprovided in the region below the lower space 18, by means of which it ispossible to cool this region, that is, to lower its temperature,relative to the remaining region of the chamber.

[0024] A heater 21 is provided in the region of the dividing wall 17,which allows heating of the region of the metallic gallium to a highertemperature, for instance to about 500° C. In this way, it is alsopossible to generate a temperature gradient, for instance from 350° C.to 450° C., between then region of the liquid gallium and theevaporation substance. This results in the evaporation substanceremaining in the solid state and not evaporating rapidly. It isunderstood that the “cooling arrangement” 20 can of course also be atype of heater, by means of which it is possible to heat the regionslocated proximate thereto to a lower temperature than the temperature inthe region of the dividing wall 17.

[0025] As further shown in FIG. 4, an additional cooling arrangement 22is provided in the region of the vapor exit opening 15, by means ofwhich it is possible to cool this region. This also can be a heater thatresults in a lower heating in relative terms. In this way, it is alsopossible to locally lower the vapor pressure prevailing in this region,so that the expansion of the dopant vapor stream emerging through theexit opening into the vacuum is low, preventing divergence of the vaporjet caused by expansion. Lastly, it should be noted that all heating andcooling arrangements can be controlled by the control unit 9 shown inFIG. 1.

[0026] Although modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

We claim as our invention:
 1. A method for generating a continuous vaporstream containing a compound having gallium therein in monovalent formin a vacuum coating procedure for vacuum coating a substrate, comprisingthe steps of: providing an evacuation chamber which is closed on allsides and which has a vapor stream exit opening; disposing a multivalentgallium-containing evaporation substance, selected from the groupconsisting of evaporation substances containing gallium in bivalent formand evaporation substances containing gallium in trivalent form, in saidevaporation chamber; disposing metallic gallium in said evaporationchamber; evaporating said evaporation substance in said evaporationchamber and thereby generating a vapor which comes into contact withsaid metallic gallium, for reducing said multivalent gallium tomonovalent gallium and causing a continuous vapor stream, containingsaid monovalent gallium, to exit from said evacuation chamber via saidvapor exit opening.
 2. A method as claimed in claim 1 comprisingproviding a first level disposed below a second level in said evacuationchamber, and disposing said evaporation substance at said first level insaid evacuation chamber and disposing said metallic gallium at saidsecond level in said evacuation chamber, and thereby evaporating saidevaporation substance separately from said metallic gallium.
 3. A methodas claimed in claim 2 comprising producing a temperature gradient from300° C. to 500° C. in said evacuation chamber between said metallicgallium and said evaporation substance.
 4. A method as claimed in claim3 comprising producing said temperature gradient from 350° C. to 450° C.5. A method as claimed in claim 3 comprising producing said temperaturegradient by cooling said evaporation substance.
 6. A method as claimedin claim 3 comprising producing said temperature gradient by heatingsaid metallic gallium.
 7. A method as claimed in claim 1 comprisingmixing said evaporation substance with said metallic gallium in saidevacuation chamber and evaporating said evaporation substance mixed insaid metallic gallium.
 8. A method as claimed in claim 1 comprisinggenerating a temperature in said evacuation chamber in a range between250° C. and 950° C. for reducing said multivalent gallium to saidmonovalent gallium.
 9. A method as claimed in claim 1 comprisinggenerating a temperature in said evacuation chamber in a range between300° C. and 900° C. for reducing said multivalent gallium to saidmonovalent gallium.
 10. A method as claimed in claim 1 comprisingcooling said evaporation chamber in a region surrounding said vapor exitopening.
 11. A method as claimed in claim 1 comprising providing a vaporexit opening in said evacuation chamber having a diameter in a rangebetween 2 μm and 2 mm.
 12. A method as claimed in claim 1 comprisingproviding a vapor exit opening in said evacuation chamber having adiameter in a range between 5 μm and 1.5 mm.
 13. A method as claimed inclaim 1 comprising providing a vapor exit opening in said evacuationchamber having a diameter in a range between 10 μm and 1 mm.
 14. Amethod as claimed in claim 1 comprising forming said evaporation chamberof a material which does not react with, and which is not wetted by,said evaporation substance.
 15. A method as claimed in claim 1comprising forming said evaporation chamber of a material which does notreact with, and which is not wetted by, said metallic gallium.
 16. Amethod as claimed in claim 1 comprising forming said evaporation chamberof a material which does not react with, and which is not wetted by,said evaporation substance nor with said metallic gallium.
 17. A methodas claimed in claim 1 comprising forming said evaporation chamber of amaterial selected from the group consisting of graphite, aluminum oxideand boric nitride.
 18. A method as claimed in claim 1 comprisingselecting said evaporation substance from the group consisting ofgallium bromide compounds and gallium indium compounds.
 19. A method asclaimed in claim 1 comprising evaporating a coating material for saidsubstrate in said evacuation chamber at a time selected from the groupof times consisting of before evaporation of said evaporation compound,during evaporation of said evaporation compound, and after evaporationof said evaporation compound.
 20. A method as claimed in claim 19comprising employing x-ray absorber material as said coating material.21. An evacuation chamber for a vacuum coating process comprising: aninterior volume for accepting an evaporation substance and at least oneadditional substance which chemically acts on a vapor produced by saidevaporation substance, said volume being closed on all sides and havinga vapor exit opening at one side, said vapor exit opening having adiameter of less than 2 mm.
 22. An evaporation chamber as claimed inclaim 21 wherein said vapor exit opening has a diameter of less than 1mm.
 23. An evaporation chamber as claimed in claim 1 further comprisinga vapor-permeable dividing wall dividing said volume into a lower spacefor accepting said evaporation substance and an upper space foraccepting said additional substance.
 24. An evaporation chamber asclaimed in claim 23 wherein said dividing wall comprises a wire grating.25. An evaporation chamber as claimed in claim 23 wherein said wallcomprises a shelf having a plurality of openings therein.
 26. Anevaporation chamber as claimed in claim 23 wherein said dividing wallconsists of a material which does not react with, and which is notwetted by, said additional substance.
 27. An evaporation chamber asclaimed in claim 23 wherein said dividing wall consists of a materialwhich does not react with, and which is not wetted by, said vapor. 28.An evaporation chamber as claimed in claim 23 wherein said dividing wallconsists of a material which does not react with, and which is notwetted by, said additional substance nor said vapor.
 29. An evaporationchamber as claimed in claim 23 wherein said upper space receivesmetallic gallium, and wherein said dividing wall comprises a materialselected from the group consisting of graphite and aluminum oxide. 30.An evaporation chamber as claimed in claim 21 further comprising aheater for heating a region of said volume.
 31. An evaporation chamberas claimed in claim 30 further comprising a base disposed at a bottom ofsaid volume, and wherein said heater is disposed in said base.
 32. Anevaporation chamber as claimed in claim 30 further comprising avapor-permeable dividing wall disposed in said volume and dividing saidvolume into an upper space and a lower space, and wherein said heater isdisposed at a region of said dividing wall.
 33. An evaporation chamberas claimed in claim 21 further comprising a cooling arrangement forcooling a region of said evaporation chamber.
 34. An evaporation chamberas claimed in claim 33 wherein said cooling arrangement is disposed forcooling an upper region of said evacuation chamber.
 35. An evaporationchamber as claimed in claim 34 wherein said cooling arrangement isdisposed for cooling a region surrounding said vapor exit opening. 36.An evaporation chamber as claimed in claim 33 wherein said coolingarrangement is disposed for cooling a lower region of said evaporationchamber.
 37. An evaporation chamber as claimed in claim 21 comprising alid-like cover closing said volume and containing said vapor exitopening.
 38. An evaporation chamber as claimed in claim 21 wherein saidwalls are comprised of a material selected from the group consisting ofgraphite, aluminum oxide and boric nitride.
 39. A vacuum coatingapparatus comprising: a vacuum chamber containing a substrate to becoated; and an evaporation chamber disposed in said vacuum chamber, saidevaporation chamber containing a multivalent gallium-containingevaporation substance and metallic gallium, and said evaporation chamberhaving a heater arrangement for evaporating said evaporation substanceto produce a vapor which comes into contact in said evaporation chamberwith said metallic gallium to reduce said multivalent gallium, saidevacuation chamber having a vapor exit opening and being otherwisecompletely closed so that a vapor stream exits said vapor exit openingin a direction toward said substrate, said vapor stream containing saidmonovalent gallium.